What Is RDW in a Blood Test?

RDW stands for Red Cell Distribution Width. It measures how much your red blood cells vary in size. A high RDW means your red blood cells are irregular in size (anisocytosis), which can help distinguish between types of anemia that have similar MCV values.

Normal Range

• Adult normal: 11.5–14.5%
• Higher values = greater variation in red blood cell size.

High RDW + Low MCV

This combination strongly suggests iron deficiency anemia (as opposed to thalassemia trait, where MCV is low but RDW is usually normal).

High RDW + Normal or High MCV

Common in combined nutritional deficiencies (both iron and B12/folate), early B12/folate deficiency before MCV rises, myelodysplastic syndromes, and chronic liver disease.

Why It Matters

RDW is especially helpful when MCV is borderline normal. A high RDW can be an early sign of iron deficiency before anemia develops (low hemoglobin). It can also predict outcomes in heart failure and other chronic conditions, and is often flagged during a routine CBC to prompt further investigation.